The present invention relates to a rolling bearing provided with an outer ring having an outer circumferential surface which has been subjected to surface hardening and which comes in contact with other members so that the outer ring is used as a roll.
In a steelworks or the like, there is a number of equipments or instruments using rolling elements such as a sintering machine pallet roller, a backup roller in a Sendzimir rolling mill, a chain conveyor, and so on. In these equipments or instruments, an outer ring of a bearing is used as a roll surface of a roller. In a steelworks or the like, works of various materials and temperatures are worked, so that such an outer circumferential surface of an outer ring of a bearing which comes in direct contact with the works is apt to be damaged conspicuously. Therefore, the outer circumferential surface of the outer ring of the bearing is subjected to surface treatment so as to make the outer circumferential surface have a hardness not less than a predetermined level.
However, a large load is often applied to the outer ring used as a roller, and on occasion the outer rings is broken by large shock loading or the like.
In a conventional bearing, carburized steel which is not larger than 0.23% in C% is used in the outer ring in order to prevent the outer ring from being broken, and the hardness in its core portion is made so as to be not larger than 500 in terms of Vickers hardness (Hv). Therefore, there is a problem that the outer ring is subjected to a plastic deformation so as to expand when a force exceeding the yield stress of the core portion acts on the outer ring.
In addition, about 15% to 30% by volume of retained austenite is left in the surface of carburized steel which has been ordinarily carburized, quenched and tempered. When the outer ring comes into contact with other mechanical elements, the retained austenite in the contact portion of the outer ring is decomposed so as to be transformed into martensite. The thus transformed martensite portion expands to cause a deformation such as a warp in the outer ring. There is a problem that this expansion deformation (warp) of the outer ring causes a deviation in abutment between an orbital surface of the outer ring and a rolling element. Particularly, when the outer ring bites a foreign matter such as sand or the like in its contact portion of the outer circumferential surface portion contacting with other mechanical elements, the decomposition of the retained austenite tends to be further accelerated. As a result, the life of the bearing becomes very short. Therefore, customers have made a strong request for the prolongation of the lives of bearings of this type.